Measurement of the jet mass distribution and top quark mass in hadronic decays of boosted top quarks in pp collisions at $\sqrt{s} = 13$ TeV

A measurement of the jet mass distribution in hadronic decays of boosted top quarks is presented. The data were collected with the CMS detector at the LHC in pp collisions and correspond to an integrated luminosity of 138$~\mathrm{fb}^{-1}$. The measurement is performed in the lepton+jets channel of $\mathrm{t}\overline{\mathrm{t}}$ events, where the lepton is an electron or muon. The products of the hadronic top quark decay are reconstructed using a single large-radius jet with transverse momentum greater than $400~\mathrm{GeV}$. The differential $\mathrm{t}\overline{\mathrm{t}}$ production cross section as a function of the jet mass is unfolded to the particle level and is used to extract the top quark mass $m_\mathrm{t}$. The jet mass is calibrated using the hadronic W boson decay within the large-radius jet. Uncertainties in the modelling of the final state radiation are reduced by studying angular relations of the jet substructure. These developments lead to a significant increase in precision with respect to an earlier measurement, resulting in $m_\mathrm{t} = 172.76 \pm 0.81~\mathrm{GeV}$.